Organic cation (OC) transporters play important roles in the disposition and clearance of many endogenous and foreign OCs in the body. In contrast to the kidney and liver, little is known about OC elimination mechanisms in the brain. This is rather unfortunate because many CNS active compounds, including monoamine neurotransmitters (e.g. dopamine, serotonin), CNS drugs (e.g. amantadine, nicotine) and neurotoxins (e.g. 1-methyl-4-phenylpyridinium (MPP+)) are small hydrophilic OCs that rely on transporters to regulate their brain levels. This proposal focuses on plasma membrane monoamine transporter (PMAT), a novel brain OC transporter first cloned in our laboratory. While structurally related to the equilibrative nucleoside transporter family (SLC29), PMAT possesses a unique and surprisingly diverse substrate specificity, transporting structurally heterogeneous OCs such as biogenic amines, clinically used drugs and neurotoxins. In humans and rodents, PMAT is most abundantly expressed in the brain and highly concentrated in the blood-cerebrospinal fluid (CSF) barrier (i.e. BCSFB or choroid plexus). We hypothesized that PMAT is the principal OC transporter at the BCSFB and is responsible for removing a variety of endogenous and xenobiotic OCs from the brain. Three Specific Aims (SAs) have been proposed. SA1 is focused on elucidating the molecular mechanisms governing transporter-substrate interactions to explain the unique and versatile substrate specificity of PMAT. SA2 is focused on elucidating the transport mechanism of PMAT and developing a cellular model for OC flux at the BCSFB. Lastly, in SA3, we will construct and validate a transgenic animal model, which will allow a variety of mechanistic studies to reveal the in vivo action of PMAT. We will apply several experimental methods, ranging from molecular biochemical techniques, computational biology, electrophysiology, immunohistochemistry to transgenic approach, to elucidate the structure, function and biological significance of PMAT in clearing neurotransmitters, drugs and toxins in the brain. The proposed studies have broad implications in our understanding of normal and pathophysiological functions of the brain. Detailed knowledge of OC transporters at the CNS barriers can also help to explain and predict the pharmacokinetics and pharmacodynamics of OC drugs and toxins in the CNS compartment and aid in the development of new strategies for drug targeting to the brain.